The invention is directed generally to an electrochemical apparatus for oxidation or consumption of a fuel, and the generation of electricity, such as, a solid electrolyte fuel cell.
Although particular embodiments are applicable to conventional co-fired solid electrolyte fuel cell apparatus, the present invention is particularly useful when utilizing non-cofired solid oxide electrolyte fuel cells, preferably planar fuel cells, that contain a stack of multiple assemblies. Each assembly comprises a solid electrolyte disposed between a cathode and an anode, being bounded by separators, which contact the surfaces of the electrodes opposite the electrolyte.
The fuel cell operates by conducting ions through the electrolyte. For solid oxide fuel cells in particular, oxygen or air is introduced at the cathode, and ionization of oxygen occurs at the cathode/electrolyte surface. The oxygen ions move across the gas non-permeable electrolyte to the anode interface, where it reacts with the fuel flowing into the anode at the anode/electrolyte interface, releasing heat and supplying electrons to the anode. Distribution of the air and fuel reactants is typically performed by a manifold assembly within the fuel cell apparatus.
Conventionally, each reactant is supplied through a flow conduit to the appropriate electrode, and distribution to the electrode/electrolyte interface is accomplished by internal porosity and/or grooved channels.
Minh, U.S. Pat. No. 5,256,499, discloses a monolithic fuel cell having an integrally formed manifold constructed by corrugations formed within the anode and cathode with aligned ribs and columns arranged to force fuel and oxidant along aligned pathways. Reactants are fed from the sides of the fuel cell and travel along these pathways.
Hsu, U.S. Pat. No. 5,747,485, discloses a conductor plate for a solid oxide fuel cell with ridges extending therefrom. These ridges form grooves used to channel reacting gases out of the cell.
Datta, U.K. Patent No. 2,219,125A discloses an electrolyte with a three-dimensional groove arrangement used to control hot spots within the electrolyte block.
Hsu, Minh and Datta employ external manifolding and rectangular geometries driving the reactants from one side of the cell to the other. Despite the use of channels, reactants entering from a single side of the cell deplete as they travel across the cell. Further, when reactants are fed externally from more than one side, the flows converge creating localized areas of increased reaction. The increased number of reactions generates an undesirable thermal gradient, which can damage the cell.
Moreover, Hsu, Minh and Datta employ grooves of uniform cross section along the length of these grooves. These grooves are essentially pathways within the cell, and fail to control gas flow rate or pressure distribution. The flow rate is controlled at its source and not tailored or controlled within the cell.
In fuel cells which have their anode fuel-exit edges exposed to an oxidizing environment, any anode local exit regions having low fuel mixture velocities may allow oxygen back diffusion into the cell stack, causing premature combustion and loss of active anode area. The electrochemical processes inherent in the fuel cell's operation become less effective and performance suffers.
Custom flow pattern design is desirable to achieve substantially uniform reactant concentration distribution within the cell and from cell to cell within a stack, which also helps minimize unnecessary and undesirable thermal gradients within the cell.
It is desirable to provide a compact, centrally fed radial fuel cell utilizing micro-channels to tailor the flow distribution of reacting gases within the fuel cell and amongst all the cells in a stack.
It is also desirable to provide a compact fuel cell utilizing variable cross-section micro-channels to tailor the flow, pressure, and velocity distribution of reacting gases within the fuel cell and amongst all the cells in a stack.
It is also desirable to provide an enhanced flow electrode produced by simple scalable production techniques.